The present invention relates to a washing method for distillation system of nitrogen-containing polymerizable monomers. Specifically the present invention relates to a method of efficiently removing the polymer formed in the distillation system and/or at the surface of packings when nitrogen-containing polymerizable monomers are distilled.
Polymers excellent in properties such as water-solubility, reactivity etc. are obtained from nitrogen-containing polymerizable monomers such as ethyleneimine, vinylpyrrolidone, allylamine, vinylformamide etc. and said polymers have been used in many applications. Nitrogen-containing polymerizable monomers are produced by various processes but the products contain, besides the monomers, unreacted starting materials and by-products, so it is necessary to separate and purify the monomers and to recover unreacted starting materials. The separation, purification and recovery have been conducted usually by distillation.
As nitrogen-containing polymerizable monomers, however, have high reactivity, polymerization of monomers themselves and polymerization of monomers with starting materials or by-products often occur during distillation and the polymer stuck to the inside of the distillation system and/or on the surface of packings provokes various problems such as lowering of separation performance, increase of pressure loss etc. of the distillation tower. In order to avoid such polymerization at the time of distillation there have been used radical polymerization inhibitors such as phenothiazine, hydroquinone etc. or various stabilizers. After operation for a long time, however, formation of polymer is observed and its removal is necessary. Therefore the polymer used to be removed by a mechanical method such as scrubbing with a brush etc.
The polymer, however, is tightly stuck to the inside of the distillation system and/or on the surface of packings, it is not easy to remove it by a mechanical method. Therefore, washing has required a long time and the productivity has been remarkably decreased due to the shutdown of the production plant during the washing.
The purpose of the present invention is to solve the above-mentioned problems and to provide a method of efficiently removing the polymer stuck in the distillation system and/or on the surface of packings when nitrogen-containing polymerizable monomers are distilled.
According to the research by the present inventors it was found that the polymer can be easily removed by washing treatment with an acidic liquid. The present invention was completed based upon such knowledge.
Thus, according to the present invention, a washing method for distillation system of nitrogen-containing polymerizable monomers characterized by washing the inside of the distillation system of nitrogen-containing polymerizable monomers and/or packings with an acidic liquid is provided.
xe2x80x9cDistillation systemxe2x80x9d in the present invention includes distillation tower, equipment attached to distillation tower, for example, heat exchanger, piping which connects them etc. Distillation tower is not particularly restricted but may be anything generally used and may be packed with packings or not. According to the method of the present invention the polymer formed in such distillation tower, heat exchanger, piping, packings etc., when products containing nitrogen-containing polymerizable monomers or crude nitrogen-containing polymerizable monomers are purified, can be efficiently removed.
xe2x80x9cNitrogen-containing polymerizable monomersxe2x80x9d in the present invention means a monomer containing nitrogen that easily forms polymers at its purification by distillation. As specific examples, there can be mentioned N-alkenyl-carboxylic acid amides such as N-vinylformamide, N-vinylacetamide, N-vinylpyrrolidone etc.; nitrogen-containing cyclic vinyl compounds such as N-vinylpyridine, 4-vinylpyridine, 2-vinylpyridine, N-vinylpyrrole, N-vinylimidazole, N-vinylmorpholine etc.; aminoacrylates such as dimethylaminoethyl acrylate, diimethylaminoethyl methacrylate etc.; acrylamides such as acrylamide, methacrylamide, methylene-bis-acrylamide, N-methylolacrylamide, diacetoneacrylamide etc.; vinyloxazolines such as 2-vinyl-2-oxazoline, 2-isopropenyl-2-oxazoline etc.; alkyleneimines such as ethyleneimine, 2-methylethyleneimine, 2-ethylethyleneimine, N-alkylazilydine etc.
xe2x80x9cAcidic liquidxe2x80x9d in the present invention is at least one acid itself selected from inorganic acids and organic acids (in case the acid is liquid) or its solution (usually aqueous solution). As specific examples of inorganic acids there can be mentioned mineral acids such as hydrochloric acid, sulfuric acid, nitric acid, phosphoric acids (orthophosphoric acid, metaphosphoric acid, pyrophosphoric acid) etc. As specific examples of organic acids there can be mentioned carboxylic acids such as formic acid, acetic acid, citric acid, glycolic acid, lactic acid, succinic acid, malic acid, oxalic acid, gluconic acid etc.; sulfonic acids such as methanesulfonic acid, p-toluenesulfonic acid, dodecylbenzenesulfonic acid etc.; phosphonic acids such as 1-hydroxyethylidene-1,1-diphosphonic acid, aminotrimethylenephosphonic acid etc. Among these acids, carboxylic acids having volatility, for example, acetic acid, are preferably used. As they evaporates without remaining in the distillation system and/or on the surface of packings, the inside of the distillation system and/or the surface of packings can be prevented from corrosion by a remaining acid. In case of using an aqueous solution its acid concentration can be suitably selected in such a range as a removing effect of polymer is obtained. Among the acidic liquid of the present invention, acetic acid and its aqueous solution are particularly preferably used.
In the method of the present invention it is preferable to wash the distillation system with water, after the above-mentioned washing with an acidic liquid, in order to remove the acid remaining in the distillation system and/or on the surface of packings.
According to the method of the present invention, the polymer formed and stuck, in the inside of distillation tower, heat exchanger, piping etc. or on the surface of packings, in case the distillation tower is packed with said packings, when products containing nitrogen-containing polymerizable monomers or crude nitrogen-containing polymerizable monomers are purified, can be removed by washing with an acidic liquid. Temperature of washing is in the range of 0-200xc2x0 C., preferably 0-150xc2x0 C. If the temperature is too high, the corrosion of equipment is severe. If, on the other hand, the temperature is too low, the washing power decreases and the operability is poor.
The method of the present invention can be carried out according to various modes. For example, {circle around (1)} in case the distillation tower is packed with said packings, the polymer stuck in the distillation tower and on the surface of packings can be washed and removed by introducing an acidic liquid into the inside of the distillation tower as it is, without taking out the packings of the distillation tower. Further, {circle around (2)} the polymer stuck on the surface of packings can be removed by washing the packings with an acidic liquid after taking them out of the distillation tower. (Usually both the inside of the distillation tower and packings are washed. According to the extent of polymer sticking, however, only inside of the distillation tower or only packings may be washed.) Moreover, {circle around (3)} in case of heat exchanger, piping etc. the polymer stuck on their inner surface can be removed by introducing an acidic liquid into all of them or each individually.
The above-mentioned washing with an acidic liquid can be conducted, for example, in case of a distillation tower, in various methods such as flushing, circulation of bottom liquid to the tower top, distillation or total refluxing in supplying the distillation tower with an acidic liquid. The above-mentioned washing with water can be conducted in a similar manner.
In case of packings the polymer can be removed more efficiently by combining the above-mentioned washing with a mechanical washing by using a brush etc.
According to the method of the present invention the polymer stuck in the distillation system and/or on the surface of packings can be efficiently removed. The productivity is improved by shortening time of the removal operation compared with the operation by using the conventional mechanical method.